Starmail is an in-game mechanic found through the Ranch House menu by clicking the "Starmail" button. It allows the player to read any Starmail the player receives over the course of the game. Any unread Starmail is automatically considered read once opening it.
Since Starmail is the trigger for some of the major features unlocked during the course of the game, reading Starmail is required to fully progress in the game.
Starmail is received when certain requirements have been met. If a certain starmail is in a series of starmails, such as Rancher introduction Starmails, it must be read before being able receive the next one. Certain Starmail act as triggers to unlock new features, such as the 7Zee Rewards Club, so must be opened to be able to receive them.
While Starmail does not allow the player to directly respond to received starmail, some of Casey's responses appear to be to a previous conversation, which implies that Beatrix responds to them when reading them.
Received 24 hours after opening your first Slime Gate, or 24 hours after reading My pet cactus if you have already opened a Gate.
Received 24 hours after opening two Slime Gates, or 24 hours after reading RE: The big storm if you have already opened two.
Received 24 hours after opening three Slime Gates, or 24 hours after reading The death hike if you have already opened three.
Received 24 hours after opening the Ancient Ruins plort gate, or 24 hours after reading RE: the... VAN if you have already opened it.
Received 24 hours after opening four Slime Gates, or 24 hours after reading New Year's Eve if you have already opened four.
Received 24 hours after opening five Slime Gates, or 24 hours after reading Re: Africa if you have already opened five.
Received 24 hours after unlocking the Glass Desert, or 24 hours after reading Coming home if you have already unlocked it.
Received 24 hours after reading Re: London.
Received 6 hours after reading Hobson's last note in The Glass Desert, or 6 hours after reading Life is different now if it has already been read; triggers the End Credits when you Return to Ranch.
Received within 10 in-game minutes when loading or starting a new save after purchasing the paid Secret Style PackDLC.
1.2.0 - Mochi will invite you to her Manor after you successfully complete a Range Exchange request with her, unlock the Indigo Quarry, and purchase The Grotto ranch expansion. Please note that you will need to complete a NEW Range Exchange request from Mochi in order to trigger her contacting you.
1.1.0 - Ogden will send you a Starmail inviting you to his retreat after you successfully complete at least one Range Exchange with him, unlock the Moss Blanket, and unlock the Overgrowth.
1.0.1 - Starmail popups now disappear once you’ve read the mail.
1.0.0 - Added the final Hobson journal nodes and Casey Starmail to Adventure Mode. Reading both is required for ‘completing’ Adventure Mode.
0.6.0 - Starmails from Casey have been overhauled to reduce feelings of clinginess, be less dramatic, and fit more with the tone of the game. However, the conclusion to the story remains unchanged. Another two Starmail messages were added, totaling 10.